Papers by Author: Derek O. Northwood

Abstract: Dissolution of secondary phases during thermal treatment in cast magnesium alloys influences their engineering properties. In this study, a kinetic model based on a Kissinger-type method has been developed for describing dissolution of secondary phases in the high pressure die cast magnesium alloy AM50 during a thermally activated heating process. Also, differential scanning calorimetry (DSC) was effectively used for investigating the dissolution kinetics of secondary phases in the AM50 alloy. By fitting a kinetic model to the DSC results, the activation energy of the dissolution of the secondary phases can be determined. In parallel, the microstructure of the alloy was analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It was found that the distribution of secondary phases and the concentration of alloying elements both at the grain boundaries and in the grains play an important role in the solid-state transformation kinetics of die cast magnesium alloy AM50.

Abstract: Corrosion performances of several metallic materials (Al6061 and Al319 alloys, 304
stainless steel and grey cast iron) in the ethanol-gasoline alternative fuels were investigated. Cyclic
potentiodynamic polarization tests were used to study their corrosion behavior. Anodizing and
plasma electrolytic oxidation (PEO) techniques were used to produce oxide coatings on the Al6061
and Al319 alloys, and the corrosion properties of these coatings in the alternative fuel environments
were also evaluated. The results showed that, the 304 stainless steel, Al6061 and the coating materials
are compatible with the alternative fuels. The oxide coatings on both Al alloys provided effective
corrosion protection in the alternative fuel environments.

Abstract: The effects of heat treatment on distortion, residual stress, and retained austenite were compared for case-carburized 4320 steel, in both the austempered and quench-and-tempered condition. Navy C-ring samples were used to quantify both size and shape distortions, as well as residual stress. The austempering heat treatment produced less distortion and a higher surface residual stress. Both hoop and axial stresses were measured; the difference between them was less than seven percent in all cases. Depth profiles were obtained for residual stress and retained austenite from representative C-ring samples for the austempered and quench-and-tempered heat treatment conditions. Austempering maintained a compressive residual stress to greater depths than quench-and-tempering. Quench-and-tempering also resulted in lower retained austenite amounts immediately beneath the surface. However, for both heat treatments, the retained austenite content was approximately one percent at depths greater than 0.5 mm.

Abstract: Mg-Al-Ca alloys with 1wt.% and 2 wt.% Ca additions (AC51 and AC52) were cast by
the Permanent Mold technique. The microstructures of the as-cast Mg-Al-Ca alloys were observed
by SEM with EDS analysis. The secondary phases were mainly precipitated along the grain
boundaries and exhibited a continuous network microstructure for the AC52 alloy and a divorced
microstructure for the AC51 alloy. EDS microanalysis showed that the solute (Ca) content in the
grains of the AC52 alloy is higher than that in the AC51. A three-sided pyramidal (Berkovich)
diamond indenter was used to characterize the local nano-creep behavior at room temperature
within the α-Mg in grains. The nano-creep results showed that the AC52 alloy has better creep
resistance than the AC51 alloy at all loads at room temperature. The creep exponent n, obtained
from the indentation creep data, changes from 6.3 to 3.0 for AC51 alloy and from 6.6 to 3.2 for
AC52 alloy at a critical stress (132 MPa for the AC51 and 145 MPa for the AC52). The transition in
creep behavior at higher stresses is associated with a change in the deformation mechanisms.

Abstract: In proton exchange membrane fuel cells (PEMFCs), the bipolar plates supply the reactant
gases through the flow channels to the electrodes and serve the purpose of electrochemically
connecting one cell to another in the electrochemical cell stack. Requirements of the bipolar plate
material are: high values of electronic conductivity; high values of thermal conductivity; high
mechanical strength; impermeability to reactant gases; resistance to corrosion; and low cost of
automated production. Metallic materials meet many of these requirements but the challenge has
been in obtaining the required corrosion resistance. In the paper, six metallic materials were
investigated as potential bipolar plate materials. The results showed that the corrosion rates were
too high even for the most corrosion resistant metals (SS316L and grade 2 Ti), and that coatings
would be required.

Abstract: SAE 8620 steel is typically used in the carburized condition for powertrain applications
in the automotive industry, e.g. gears, roller bearings, camshafts. Such steels always contain
retained austenite to varying degrees in the as-hardened and also in the tempered microstructures.
As well as retained austenite, heat treatment can produce residual stresses, which lead to distortion
(size and shape). The intent of this study was to investigate the effect of heat treatment parameters
on the amount of retained austenite, residual stress and distortion in carburized SAE 8620 steel.
A specially designed specimen, the Navy C-ring, was used for this study. The steel was first
normalized prior to machining the Navy C-ring specimens. The specimens were then heat treated
by carburizing at 927°C or 954°C (1700°F or 1750°F) at four levels of carbon potential (0.9, 1.0,
1.1, 1.2) followed by oil quenching and tempering at either 149°C or 177°C (300°F or 350°F). The
distortion of the C-ring was evaluated by dimensional measurements of the inner diameter, outer
diameter, gap width and thickness for size distortion, as well as flatness, cylindricity and roundness
for shape distortion. X-ray diffraction (XRD) techniques were used to determine the residual stress
and the amount of retained austenite. The amount of retained austenite was also measured by
optical metallography. The amount of retained austenite and the residual stress increased with
increasing carburizing temperature and carbon potential and decreased upon tempering. There was
not a significant further reduction in the amount of retained austenite and residual stress when the
tempering temperature was increased from 149°C to 177°C. Distortion was influenced by both the
amount of retained austenite and the magnitude of the residual stress. With increasing retained
austenite/residual stress, the distortion became more serious. Based on the distortion data for 3
parameters (OD, gap width and flatness) for the quenched and tempered specimens, the amount of
retained austenite for minimum distortion was approximately 25%.